Another option, based on the size of whitewater you will be running, is to go with a true white water pontoon by Aire or NRS with a custom frame. Price wouldn't be that much more and I think you'd be much happier. Check out some of the white water companies such as NRS, Aire, Sotar, Maravia, Down River Equipment Company and see if you can put something together that truly will meet your needs. Think you'll be happier in the long run.
Chris